path: root/mm
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authorVladimir Davydov <vdavydov@parallels.com>2015-02-12 22:59:47 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2015-02-13 02:54:10 (GMT)
commitd6e0b7fa11862433773d986b5f995ffdf47ce672 (patch)
tree031830bb978d8861c3089941480de8effe9ccc6a /mm
parentce3712d74d8ed531a9fd0fbb711ff8fefbacdd9f (diff)
slub: make dead caches discard free slabs immediately
To speed up further allocations SLUB may store empty slabs in per cpu/node partial lists instead of freeing them immediately. This prevents per memcg caches destruction, because kmem caches created for a memory cgroup are only destroyed after the last page charged to the cgroup is freed. To fix this issue, this patch resurrects approach first proposed in [1]. It forbids SLUB to cache empty slabs after the memory cgroup that the cache belongs to was destroyed. It is achieved by setting kmem_cache's cpu_partial and min_partial constants to 0 and tuning put_cpu_partial() so that it would drop frozen empty slabs immediately if cpu_partial = 0. The runtime overhead is minimal. From all the hot functions, we only touch relatively cold put_cpu_partial(): we make it call unfreeze_partials() after freezing a slab that belongs to an offline memory cgroup. Since slab freezing exists to avoid moving slabs from/to a partial list on free/alloc, and there can't be allocations from dead caches, it shouldn't cause any overhead. We do have to disable preemption for put_cpu_partial() to achieve that though. The original patch was accepted well and even merged to the mm tree. However, I decided to withdraw it due to changes happening to the memcg core at that time. I had an idea of introducing per-memcg shrinkers for kmem caches, but now, as memcg has finally settled down, I do not see it as an option, because SLUB shrinker would be too costly to call since SLUB does not keep free slabs on a separate list. Besides, we currently do not even call per-memcg shrinkers for offline memcgs. Overall, it would introduce much more complexity to both SLUB and memcg than this small patch. Regarding to SLAB, there's no problem with it, because it shrinks per-cpu/node caches periodically. Thanks to list_lru reparenting, we no longer keep entries for offline cgroups in per-memcg arrays (such as memcg_cache_params->memcg_caches), so we do not have to bother if a per-memcg cache will be shrunk a bit later than it could be. [1] http://thread.gmane.org/gmane.linux.kernel.mm/118649/focus=118650 Signed-off-by: Vladimir Davydov <vdavydov@parallels.com> Cc: Christoph Lameter <cl@linux.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: David Rientjes <rientjes@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
5 files changed, 43 insertions, 11 deletions
diff --git a/mm/slab.c b/mm/slab.c
index 7894017..c4b89ea 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2382,7 +2382,7 @@ out:
return nr_freed;
-int __kmem_cache_shrink(struct kmem_cache *cachep)
+int __kmem_cache_shrink(struct kmem_cache *cachep, bool deactivate)
int ret = 0;
int node;
@@ -2404,7 +2404,7 @@ int __kmem_cache_shutdown(struct kmem_cache *cachep)
int i;
struct kmem_cache_node *n;
- int rc = __kmem_cache_shrink(cachep);
+ int rc = __kmem_cache_shrink(cachep, false);
if (rc)
return rc;
diff --git a/mm/slab.h b/mm/slab.h
index 0a56d76a..4c3ac12 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -138,7 +138,7 @@ static inline unsigned long kmem_cache_flags(unsigned long object_size,
int __kmem_cache_shutdown(struct kmem_cache *);
-int __kmem_cache_shrink(struct kmem_cache *);
+int __kmem_cache_shrink(struct kmem_cache *, bool);
void slab_kmem_cache_release(struct kmem_cache *);
struct seq_file;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 0873bcc..1a1cc89 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -549,10 +549,13 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
int idx;
struct memcg_cache_array *arr;
- struct kmem_cache *s;
+ struct kmem_cache *s, *c;
idx = memcg_cache_id(memcg);
+ get_online_cpus();
+ get_online_mems();
list_for_each_entry(s, &slab_caches, list) {
if (!is_root_cache(s))
@@ -560,9 +563,17 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
arr = rcu_dereference_protected(s->memcg_params.memcg_caches,
+ c = arr->entries[idx];
+ if (!c)
+ continue;
+ __kmem_cache_shrink(c, true);
arr->entries[idx] = NULL;
+ put_online_mems();
+ put_online_cpus();
void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
@@ -649,7 +660,7 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
- ret = __kmem_cache_shrink(cachep);
+ ret = __kmem_cache_shrink(cachep, false);
return ret;
diff --git a/mm/slob.c b/mm/slob.c
index 96a8620..94a7fed 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -618,7 +618,7 @@ int __kmem_cache_shutdown(struct kmem_cache *c)
return 0;
-int __kmem_cache_shrink(struct kmem_cache *d)
+int __kmem_cache_shrink(struct kmem_cache *d, bool deactivate)
return 0;
diff --git a/mm/slub.c b/mm/slub.c
index 7fa27ae..06cdb18 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2007,6 +2007,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
int pages;
int pobjects;
+ preempt_disable();
do {
pages = 0;
pobjects = 0;
@@ -2040,6 +2041,14 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
!= oldpage);
+ if (unlikely(!s->cpu_partial)) {
+ unsigned long flags;
+ local_irq_save(flags);
+ unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
+ local_irq_restore(flags);
+ }
+ preempt_enable();
@@ -3369,7 +3378,7 @@ EXPORT_SYMBOL(kfree);
* being allocated from last increasing the chance that the last objects
* are freed in them.
-int __kmem_cache_shrink(struct kmem_cache *s)
+int __kmem_cache_shrink(struct kmem_cache *s, bool deactivate)
int node;
int i;
@@ -3381,11 +3390,23 @@ int __kmem_cache_shrink(struct kmem_cache *s)
unsigned long flags;
int ret = 0;
+ if (deactivate) {
+ /*
+ * Disable empty slabs caching. Used to avoid pinning offline
+ * memory cgroups by kmem pages that can be freed.
+ */
+ s->cpu_partial = 0;
+ s->min_partial = 0;
+ /*
+ * s->cpu_partial is checked locklessly (see put_cpu_partial),
+ * so we have to make sure the change is visible.
+ */
+ kick_all_cpus_sync();
+ }
for_each_kmem_cache_node(s, node, n) {
- if (!n->nr_partial)
- continue;
for (i = 0; i < SHRINK_PROMOTE_MAX; i++)
INIT_LIST_HEAD(promote + i);
@@ -3440,7 +3461,7 @@ static int slab_mem_going_offline_callback(void *arg)
list_for_each_entry(s, &slab_caches, list)
- __kmem_cache_shrink(s);
+ __kmem_cache_shrink(s, false);
return 0;

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